Hereinafter, the term “structures” is defined as all types of elements which are produced by various chemical and/or physical processes directly on a wafer or which are externally produced and are joined to the wafer by any alignment process or placement process.
Examples of structures which are produced directly on the wafer are vapor-deposited conductor paths, ICs produced lithographically on the wafer, for example CMOS or TTL logics, sensors, etching structures, MEMS, etc.
On the other hand, a wafer can also be provided with components by an insertion process. The most common example for an insertion process would be the application of chips in a chip-to-wafer process by a pick-and-place (i.e., transfer) device. These components represent 3D expansions of the basic structure of the wafer. These components are also encompassed by the term “structures,” as used herein.
All the aforementioned structures can have deviations from the ideal. For example, conductor paths can have slight distortions due to faults in the mask. It would furthermore be conceivable that the conductor paths have indeed been correctly produced on the wafer, but in a subsequent bond process such a high pressure is applied to the wafer that the conductor's surface and thus also the conductor paths are distorted. Furthermore distortions of the surface can arise due to other technical-physical and/or chemical influences, for example by thermal stresses, thermal shock, inherent stresses, etc. Similar considerations apply to all structures which have been applied directly to a wafer.
In structures which are applied by an insertion process to the wafer surface, the positioning and/or alignment of the structure can be faulty. In this case distortion is defined as distortion of the applied structure itself, predominantly caused by torsion and shearing.
Alignment in bond processes, especially chip-to-wafer methods, is becoming increasingly more important due to the 3D technology which is becoming more and more important in combination with advancing miniaturization. This acquires importance mainly in applications in which alignment accuracies of less than 2 μm for all sites located on the wafer are desired. The importance and demands on the accuracy of alignment technology are still increasing greatly for desired accuracies less than 1 μm, especially less than 0.5 μm, or less than 0.25 μm.
Due to the fact that the structures are becoming smaller and smaller, but the wafers at the same time are becoming larger and larger, structures which are very well aligned to one another may be present in the vicinity of alignment marks, while at other positions of the wafer the structures have not been correctly or at least not optimally placed.
For this reason, metrology tools are used for checking of alignment accuracies. EP 2299472 shows a method in which it is possible to measure the entire surface of a wafer in order to obtain information about the positions of the structures on the surface of each wafer.
The structures mentioned here can be deformed in exactly the same manner by high pressures, thermal stresses, inherent stress, thermal shocks, etc.
The object of this invention is to develop a generic device and a generic method such that checking of the alignment accuracy and/or of the distortion more efficiently and more accurately is enabled.
This object is achieved with the features of the claims. Advantageous developments of the invention are given in the dependent claims. All combinations of at least two of the features given in the specification, the claims and/or the figures also fall within the scope of the invention. At the given value ranges, values within the indicated limits will also be considered to be disclosed as boundary values and will be claimed in any combination.
In accordance with the invention, two coordinate systems are provided, namely, the X-Y coordinate system, also called the first coordinate system, and the X′-Y′ coordinate system, also called the second coordinate system.
The first coordinate system allows translational and/or rotary motion of the receiving unit and thus of the substrate holder with the wafer loaded onto it and the positioning relative to at least one, preferably several optics, which have been mounted in a fixed manner. The optics can preferably also move in translation and rotation, for calibration of the optical axis or of the optics, relative to the first coordinate system. Conversely, the receiving unit with the substrate holder can also be fixed during the determination of the alignment errors and the optics can be movable. In this case, at the start a position of the substrate holder or substrate would be fixed as the origin of the first coordinate system.
The origin of the first coordinate system lies preferably in the optical axis of the detections means, especially one of the optics.
The second coordinate system is a coordinate system which is defined in the computer and with reference to which a structure position field is defined.
The invention is based on the idea of comparing the actual X-Y positions (detected in the first coordinate system), namely, the actual alignment in the X-Y plane, of a structure which are present on the substrate, to the ideal X′-Y′ structure positions of a structure position field which has been generated in the computer, which latter positions are stored in the second coordinate system. The structure position field is preferably defined with respect to the second coordinate system, which is joined in a fixed manner to the alignment unit and the sample holder.
According to the invention, the substrate (wafer) is positioned by alignment mark (markings on the substrate) relative to the second coordinate system, preferably by translational and/or rotary motion. The structure position field is then in the ideal case congruent with the X-Y positions of the structures present on the wafer. Alternatively, a software correlation with the second coordinate system takes place so that a transformation of the two coordinate systems is possible.
Due to the alignment of the substrate by means of its alignment marks (markings) to the second coordinate system, not only are errors in the movement/assignment/detection by the detection means and their movement relative to the substrate holder or the substrate minimized, and even precluded, but also much more efficient and faster detection is enabled.
In the following description, substrates, such as wafers, and the structures, such as chips which are applied to wafers, in several layers (so-called 3DIC chips), or structures which have been produced directly on the wafer by various processes, are not mentioned in detail. Due to the independence of the X-Y structure positions assigned to the second coordinate system, the invention is especially suited for applying several layers of chips (or several layers of structures applied directly on the wafer by different processes which are not mentioned in detail) since error propagation or error multiplication is avoided by the measure as claimed in the invention.
Based on the detection in an external coordinate system (second coordinate system) (assigned to the machine/device) the method as claimed in the invention is also suited for determining the distortions of the X-Y structures on the substrate which are caused for example by stresses which are induced in wafer bonding. The method can also be used when only one substrate to be bonded has been structured. This is the case for example in the production of back side illuminated image sensors.
The two coordinate systems are especially Cartesian coordinate systems which are each determined by X vectors (X direction) and Y vectors (Y direction) which intersect at the origin of the coordinate systems.
The device as claimed in the invention has detection means, i.e., optics, preferably several optics, preferably at least a microscope and/or a laser and/or a camera. The detection means can be moved in rotation by three degrees of freedom and in translation by three degrees of freedom in order to allow calibration. Preferably, the detection means are fixed or can be fixed during the method steps, as claimed in the invention. According to the invention, the relative motion between the wafer and the optics takes place as claimed in the invention by the active movement of the receiving unit with reference to the first coordinate system.
The distance of the X-Y positions of the first and/or second markings in the X direction and/or the Y direction in the second coordinate system to the given X-Y structure positions can be determined by preferably digitized superposition of the X-Y structure position assigned to each structure with the X-Y positions of the second markings, preferably by digital image acquisition of the structure.
The first markings, so-called alignment marks, are used for coarse and/or fine alignment of the substrate, which has been fixed on the substrate holder and/or for correlation of the position of the substrate in a first coordinate system to the second coordinate system in the X and Y direction, preferably in addition in the direction of rotation. According to preferred embodiment of the inventions, solely more finely resolved second markings are used to determine the origin of the second coordinate system so that it can be determined more accurately.
According to one advantageous embodiment of the invention, the detection means comprise at least one optics which can be fixed in the first coordinate system and which can be moved at least in the X and Y direction in the first coordinate system, preferably controlled by a control apparatus, for setting the origin of the first coordinate system. The detection means can comprise a single microscope or several microscopes which can be triggered preferably independently of one another.
It is especially advantageous if the optics, especially by focusing and/or moving in one Z direction which is perpendicular to the X and Y direction, has a field of view with which at least one structure at a time can be detected at the same time, preferably less than 17 structures at the same time, even more preferably less than 5 structures at the same time, ideally exactly one structure at the same time.
In one embodiment of the invention, alignment means are provided in the form of a receiving apparatus which accommodates the substrate holder and which can moved at least in the X and Y direction of the first coordinate system. The alignment means are intended for alignment of the substrate which is fixed on the substrate holder relative to the second coordinate system, by detecting the first markings on the substrate with the detection means.
To the extent the X-Y positions and/or the X-Y structure positions can be stored jointly in a position map which is assigned especially to the second coordinate system or which is correlated with it, prompt and efficient evaluation of the structures is possible. As a result, at any instant, when several structures are stacked on top of one another, a faulty structure alignment can be ascertained and corresponding countermeasures, such as for example re-alignment or marking as scrap, can be initiated.
One important aspect of this invention is that a device (or measurement device) as claimed in the invention in one preferred embodiment is provided separately from the alignment device as an independent module.
Features disclosed according to the device and also method features should be considered disclosed as an independent or combined invention and vice versa.
To the extent the method as claimed in the invention or the device as claimed in the invention are used in BSI CIS (back side illuminated contact image sensors), the determination of distortions in an exposure field for lithography, especially with a maximum size of 26×32 mm, is important. The order of magnitude of deviations is especially less than 250 nm, preferably less than 100 nm, still more preferably less than 70 nm, even more preferably less than 50 nm.
According to one embodiment of this invention, it is conceivable for the accuracy of detection of fields of view adjacent to distortions, which are referenced to the respectively detected field of view, to be considered by interpolation or other suitable transformation methods.
Other advantages, features and details of the invention will become apparent from the following description of preferred exemplary embodiments and using the drawings.